Bath Water Question....

[Aymer de Valence]

Wasn't quite sure where this strange thread would go, so apologies if it's in the wrong place!


But anyway, I was wondering why the water in a freshly-run bath sometimes feels 'cold' because it's so hot (a comfortable temperature I mean, not boiling)? I know nothing about science (history nerd) or even if it's just me who has noticed this phenomenon, but it's a question that has bugged me for a long time.

Is it something to do with the material the bath is made out of? How the cold water has dispersed once I have run that into it? Or how my skin reacts etc? The strangest thing though is that it seems to only happen at certain times of the day...... It feels normal at 6am before work every day, but if I have a bath on a Saturday afternoon etc, that's when the water/skin feels strange....


Yeah strange question, sorry!

[G-Megas-Doux]

Temperature is actually relative. If you are warmer then the surrounding temperature then it is cold. if you are colder then the surrounding temperature then it is warm. The same applies with bath water, you only need to be warmer than the water for it to feel cold. This is the same principle why people with hypothermia feel warm.

[Caelifer_1991]

The sensors in our skin, at extreme temperature ranges, lose the ability to differentiate between the direction of a temperature change, merely the magnitude, afaik. I say this as an engineering guy, not a biology guy, but I'm sure someone more invested in the subject will be happy to come along and better define/ explain it.

[Aanker]

But anyway, I was wondering why the water in a freshly-run bath sometimes feels 'cold' because it's so hot (a comfortable temperature I mean, not boiling)? I know nothing about science (history nerd) or even if it's just me who has noticed this phenomenon, but it's a question that has bugged me for a long time.

If I understand this correctly you are referring to the sensation of the hot water slightly resembling that of touching very cold water, yes? If so, then it's likely down to the effect of pain receptors in your skin. These may be activated both by low temperatures and high temperatures, the end point of which is a burning perception regardless of whether the original stimulus was cold or hot. Bear in mind that the receptors are activated gradually. This means that even at "non-painful" levels of heat they may start to fire and, depending on other factors, you will be aware of the signal.

Otherwise, if this was not really in line with the question, I would refer to the first response. In the morning, one possible explanation is that the air where you live is cooler, so stepping into the water feels warming. On a sunday afternoon, temperatures have risen a bit and you may yourself be slightly warmer as a result of exercise or work. Thus the water feels cooling. Maybe.

[Magister Militum Flavius Aetius]

I tell you what, when you pour hydrochloric acid on your hand it goes cold for a second and burns like HELL

[Aymer de Valence]

If so, then it's likely down to the effect of pain receptors in your skin.

If you are warmer then the surrounding temperature then it is cold. if you are colder then the surrounding temperature then it is warm.

I think it must be a combination of these two things. I've always wondered what that weird sensation was lol

[Elfdude]

I tell you what, when you pour hydrochloric acid on your hand it goes cold for a second and burns like HELL

Yes. Touching acid that can corrode a steel plate is a great idea. Can you please stop posting potentially dangerous and harmful suggestions?

Since no one has answered you with the correct answer.

The reason is because of the receptors we have for touch and temperature in our hand. Not our pain receptors (although any receptor stimulated enough can trigger the sensation of pain). Cold receptors are sensitive from 65-85 degrees (farenheight) and warm receptors are sensitive from 86 to 104 degrees (F).

We have warm spots and cold spots on our hands or rather areas which are better than others at sensing colder or warmer temperatures. This leads to a local differential in the temperature we sense. Now there's a competitive inhibition that also happens. Because our hands are packed so tightly with receptors we can easily flood our brain with signals, the result of doing so is turning down the sensitivity on all of them. If the brain turns down all of the screaming heat receptors due to a sudden and dramatic increase in their activity the result is a lot of cold receptors which are active and still sending signals. This sensation is further strengthened when at extreme temperatures the receptors fire wrongly. For example, if I splash 145 degree (F) water on myself while all my warm receptors are being turned off and all of my cold receptors aren't there's also a paradoxical cold signal. Essentially speaking, the high temperatures can cause spontaneous depolarization of the membranes of the cold thermoreceptors which in turn sets off a signal cascade that is interpretted as cold.

So 3 things in turn are leading to this.
1. Competitive inhibition of heat receptors (high signal strength being down-regulated automatically)
2. Cold and warm spots which are more or less likely to give such a response (such as your hands which are super dense in both)
3. Paradoxical cold signals (triggered by depolarization caused by high temperatures)

Your receptors aren't really relative to temperatures. While your brain does turn them down say if you've been cold for awhile your brain turns off the signal from the cold receptors, the receptors themselves fire at specific ranges because that is what is evolutionarily beneficial. It would be problematic if your cold receptors and warm receptors were relative because then you wouldn't be aware if you were overheating. Because of the brains penchant for turning down high frequency signals if you're a very warm person (or are feeling warm) you're far more likely to feel paradoxical cold because the brain has already shut down some of your warmth receptors and your cold receptors have not been.

[Ulyaoth]

^so i take it that effect can become long lasting, in explaining why the present time of year with consistent sub freezing temperatures can feel less uncomfortable, provided you dress appropriately and don't get your hands or feet wet, than say late fall when it's just about freezing or warmer. Or why 50 degrees in spring feels so warm and nice compared to being cold in fall.

[Elfdude]

Yes and no, when the human body gets to a certain temperature different mechanisms also kick in. So while you might feel colder in the fall/spring than in the middle of winter your caloric intake/expenditure is also much higher in the winter. Essentially speaking our first inclination is to get more clothes to get us back to our 72 degrees F ideal summer temperature but by winter our average ideal temperature is around 60-65 degrees F, the gap is caused by dramatically increased metabolism and superior resistance to radiating heat. The radiating heat is mostly caused by restricting blood flow and less production of oils which reduce the conductivity of your skin.